Loss-in-weight feeders are in widespread use in the material handling industry and are used to deliver particulate and other materials at precise feed rates. Typically, a loss-in-weight feeder measures the decreases in weight of material contained in the feeder over a period of time. The weight loss by the feeder in that period of time is exactly equal to the weight of material delivered to the next step in the material handling process. This mode of operation, because it measures change in weight per unit time, is referred to as "gravimetric" operation.
An advantage of loss-in-weight feeders is that there is no possibility of feed rate errors due to storage or material accumulation in the material handling system. Thus, for example, if it is determined that the weight of a loss-in-weight feeder has descreased 30 pounds in one hour, then it can be said with certainty that those 30 pounds have been delivered to the next step in the process and are not "lost" due to storage or spillage somewhere in the system upstream of the feeder.
A disadvantage of present loss-in-weight feeding systems is that, in measuring weight loss from a feeder, accurate weight measurements cannot be made when material is being added to the feeder during refill. While the feeder is being refilled, loss-in-weight measurements are impossible because material is being added to the feeder at an uncontrolled rate. In addition, the weighing device, usually a scale, is subjected to impact forces generated by the added material, entrapped air, and other factors which result in weight readings that vary widely and are inaccurate. During refill, therefore, it is customary to operate loss-in-weight feeders in a volumetric mode (which delivers a given volume of material per unit time) rather than in a gravimetric mode which would produce inaccurate feed control. Because refill may constitute a substantial portion of the feeder operating cycle, the overall accuracy of the system may be significantly reduced.
The necessity for switching to a volumetric mode of operation during refill has additional disadvantages as well. In order to achieve high feed rates, refill time must be minimized. This requires expensive, high-quality, critically-damped scales to make certain that perturbations introduced during refill have been damped out when the system re-enters the gravimetric mode. If the chosen scale is under-damped or over-damped, large hoppers are required to ensure that the system will operate in gravimetric control for a long enough time to provide stable weight data. However, the use of large hoppers to accomodate high feed rates can also result in poor accuracy because larger hoppers are more easily affected by outside forces than small hoppers. In order to minimize inaccuracy, therefore, feed rates must generally be kept low.
Accordingly, the present invention has been developed to substantially reduce the foregoing problems and to produce an improved loss-in-weight feeding system which allows gravimetric operation for 100 percent of the operating time.